Southern California -- this just in

Lifeguards are urging caution after half a dozen great white shark sightings along the Southern California coast in the past week, but experts downplayed any risk of attacks on humans, saying there is no cause for alarm.

San Diego lifeguards issued a warning Sunday for two miles of coastline from La Jolla Cove to Scripps Pier after a kayaker reported seeing what appeared to be a large great white swimming off La Jolla Shores. Later that day, lifeguards spotted a shark’s 20-inch dorsal fin sticking out of the water about 50 yards from the beach.

“We didn’t close the beach, we just let people know -- the scuba divers, kayakers, and swimmers -- that there's been shark sightings and just use your own judgment on whether to go out or not,” said Maurice Luque, a spokesman for the San Diego Fire-Rescue Department.

The advisory will expire Tuesday if no additional shark sightings are verified by lifeguards.

The advisory in La Jolla is the latest in a week of shark sightings, ushered in last week when surfer Chuck Patterson shot video of a pair of great whites circling his stand-up paddle board in the water off San Onofre. The clip has garnered hundreds of thousands of viewers online and has been aired on national newscasts.

Witnesses have reported half a dozen great white shark sightings in the past week in Sunset Beach, San Onofre and La Jolla, according to the Shark Research Committee, a nonprofit that logs reports of sharks on its website.

But experts stress that uncorroborated eyewitness reports of great whites often prove to be false or a case of misidentification. Great whites attract a lot of attention, even hysteria, but attacks are exceedingly rare.

The last known fatal shark attack on the West Coast was in April 2008 when 66-year-old triathlete Dave Martin was killed while swimming near Fletcher Cove in Solana Beach. That attack, thought to be by a great white, prompted authorities to close eight miles of coast for several days.

While found in oceans worldwide, great whites are one of the least understood sharks, and researchers do not know precisely how many of them exist and their range, though satellite tracking studies are yielding some clues.

As far as researchers can tell, Southern California is a nursery ground for juvenile sharks measuring between 4 and 10 feet. Since sharks of that age hunt mostly fish, there’s little danger of the creatures mistaking swimmers for seals or porpoises, as full-grown great whites are thought to when they attack humans.

“You talk to most surfers up and down the coast and they see them on a pretty regular basis, they just don't report that because it's a pretty regular occurrence and they don't think much of it,” said Andrew Nosal, a shark researcher at the Scripps Institution of Oceanography.

While Nosal recommends being cautious, and staying out of the water if you feel unsafe, “just the fact that we're seeing a shark in the water is no reason to be alarmed,” he said.

“It's cliche, but it's true that you have more of a chance of being killed on a freeway on the way to the beach than being killed by a shark at the beach,” he said.